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Why use crystalloids?
The advantages of crystalloid solutions are:
- easy to store with long shelf life
- readily available
- very low incidence of adverse reactions
- a variety of formulations are available
- effective for use as replacement fluids or maintenance fluids
- no special compatibility testing is required.
- no religious objections to their use
In essence they are cheap and effective and don't cause adverse reactions.
Crystalloids solutions are classified into three groups based on their predominant use. The contents of the various solutions are listed in the table below.
These solutions are used to replace ECF. They are all isotonic. The key factor is that these solutions have a [Na+] similar to that of the extracellular fluid which effectively limits their fluid distribution to the ECF. The fluid distributes between the ISF and the plasma in proportion to their volumes. Intracellular fluid volume does not change. If used to replace blood loss, 3 to 4 times the volume lost must be administered as only 1/3 to 1/4 remains intravascularly. In healthy adults with a normal initial haemoglobin level, up to 20% loss of blood volume (loss of approx 1,000 mls) can be safely replaced with a 3,000-4,000 ml infusion of replacement solution without any adverse effects.
Hartmann's solution contains lactate as a bicarbonate precursor. The metabolism of lactate in the liver results in production of an equivalent amount of bicarbonate. Similarly, Plasmalyte 148 solution contains acetate and gluconate as bicarbonate precursors. These anions (eg lactate) are the conjugate base to the corresponding acid (eg lactic acid) and do not contribute to development of an acidosis as they are administered with Na+ rather than H+ as the cation.
These solutions are used to provide maintenance fluids. They are isosmotic as administered and do not cause haemolysis. Following administration, the glucose is rapidly taken up by cells so the net effect is of administering pure water. Dextrose 5% contains no Na+ so it is distributed throughout the total body water with each compartment receiving fluid in proportion to its contribution to the TBW. (See Section 8.1).
Some maintenance solutions also have Na+ so they can be prescribed to provide the daily maintenance requirements for water and Na+. Supplemental K+ can be added as required. The normal daily Na+ intake of 1.5 to 2 mmol/kg can be given in this way by appropriate fluid selection. The Na+ content does alter the fluid distribution but this is predictable.
Hartmann's solution contains Ca2+ and this can cause problems if administered with stored blood. Citrate is the anticoagulant used in stored blood and it works by complexing with and removing Ca2+ from solution. It is possible for the Ca2+ in Hartmann's to cause clotting of blood in the infusion tubing particularly if the blood is given slowly or the tubing contains reservoir areas (eg as in pump sets). For this reason, it has become standard practice to administer normal saline before and after a blood transfusion to prevent blood and Ca2+ mixing in the infusion tubing. Plasmalyte 148 solution contains Mg2+ instead of Ca2+ and can be administered with stored blood without causing this problem.
Some crystalloid solutions used for special purposes are grouped together here, for example:
- Hypertonic (3%) saline
- Half normal saline
- 8.4% Bicarbonate solution
- Mannitol 20%